Improved titanium alloy capacitor

ABSTRACT

This invention relates to an improved electrical capacitor comprising, as an electrode, a binary alloy of titanium and aluminum or ternary alloy of titanium, zirconium and aluminum, said electrode having an anodic dielectric oxide film formed thereon. The novel capacitors have high capacitance and low leakage current at high voltages.

United States Patent [56] llelerencel Cited UNITED STATES PATENTS1,889,415 11/1932 Mershon 317/230 2,299,228 10/1942 Gray et al. 1317/230 2,504,178 4/1950 Burnham et al 317/230 3,166,693 1/1965 Haringet a]. 317/230 3,182,235 5/1965 Mahler et a1... 317/230 3,330,999 7/1967317/230 Primary Examiner-James D. Kallam Attorney-Wenderoth, Lind andPonack ABSTRACT: This invention relates to an improved electricalcapacitor comprising, as an electrode, a binary alloy of titanium andaluminum or ternary alloy of titanium, zirconium and aluminum, saidelectrode having an anodic dielectric oxide film formed thereon. Thenovel capacitors have high capacitance and low leakage current at highvoltages.

I i l i l l cllrlllnwlll III PATENTEUAIJBIOIQTI 3,599,053

SHEEI 2 0F 2 KOREAKI NAKA'I'A and (051110 IIDA, Inventnrr;

Attorneys IMPROVED TITANIUM ALLOY CAPACITOR Electrical capacitors.especially those of the electrolytictype, commonly employ metalelectrodes on which a thin dielectric oxide coating has been formed.l-leretofore, aluminum and tantalum have been commonly employed ascapacitor electrode materials, and while each material has been foundparticularly useful under certain conditions, they have certaindrawbacks. Aluminum is light weiflit and superior in the ductility whichfacilitates making a thin foil. In addition, aluminum foil u eailyincreased in the surface area thereof by surface etching, and is lessexpaisive than tantalum. Therefore, aluminum in a foil form has beenused widely for making electrolytic capacitor in a wet-type. n the otherhand, tantalum forms a dielectric oxide film thereon which is superiorin the electrical properties, especially in the leakage current at highvoltages and easily makes a porous body compared to aluminum. However,tantalum is more expensive than aluminum and is restricted in theapplication.

Titanium has also been considered heretofore for possibleusemacapacitorelectrodematerialinviewofthehigh dielectric constant ofits oxide and other good properties such as corrosion resistance and lowdensity. However, it has been difficult to make a dielectric oxide filmon the surface of titanium superior in leakage current at high voltages.Such drawback of titanium prevents a wide use for an electrode adaptedto an electrolytic capacitor in spite of its high dielectric constantand good sinterability. Especially, a porous body of titanium is knownto be inferior in respect of the leakage current when formed into anelectrolytic capacitor of the solid type.

An object of the present invention is to provide an electrolyticcapacitor characterized by a low leakage current at high voltages.

Another object of the invention is to provide a titanium alloyelectrolytic capacitor of a solid t'ype characterized by a largecapacitance and a low leakage current at high voltages.

A further object of the present invention is to provide a method formaking an electrolytic capacitor characterized by a low leakage currentat high voltages.

These and other objects of the invention will be evident uponconsideration of the following detailed description taken together withaccompanying drawings wherein:

FIG. 1 isacron-wctionalviewofanelectrolyticcapacitor of the wet-type inaccordance with the invention.

FIG. 2 is a cross sectional view of the electrolytic capacitor of thesolid type in accordance with the invention.

FIG. 3 is a triangular compositional diagram of electrode materialsutilized in the present invention.

Before proceeding with the detailed description of the novel electrodeand its preparation method contemplated by the invention, a constructionof an electrical capacitor comprising said electrode will be describedwith reference to FIGS. 1 and 2 of the drawing.

Referring to FIG. 1, a reference character 30 indicates, as a whole, awet-type electrolytic capacitor comprising a container 4 serving as thecathode and containing an electrolyte 3 inwhichanaaodelisimmersed.Saidanode lconsistsof titanium alloy according to theinvention and is formed into a desired form and is anodically oxidizedin a manner illustrated in detail here'aiaf'ter. In this embodiment,said anode l is a sintered body of titanium alloy prepared in a mannerdescribed hereinafter. A lead wire 2 made of a film forming metal suchas the same alloy as the anode, or of niobium, titanium, zirconium,tantalum or other film-forming metal, is embedded in said sintered body1 oftitanium alloyandextendstoan insulating plug 5 made of anyinsulating and watertight material such as rubber or resin. Electricleads 6 and 7 made of any solderable metal such as copper or iron areattached to said filmforming metal lead wire 2 and said container 4 in aper se wellknown method such as welding method.

Referring to FIG. 2. reference character 40 indicates a base electrodeI. Said base electrode 1 may be of any form such as plate, wire orsintered form prepared in a manner illustrated hereinafier. Saidsintered body I has a film-forming metal lead wire 2 embedded therein.Said sintered body and film-forming metal lead wire are coated withoxide film ll by being anodically oxidized in a manner illustratedhereinafter. Said filmforming metal lead wire 2 is made of the samealloy, or of niobium, titanium, zirconium, tantalum or otherfilm-forming metal. Said oxide film 11 is coated with a layer 8 of asemiconductive material in a per se well-known method. A carbon filmlayer 9 is integrated onto said layer B of semiconductive material byper se a conventional method such as a brushing method in which a carbonink is applied by brushing. A silver paint it) as counterelectrode isapplied to said carbon film layer 9. Electric leach 6 and 7 made of anysolderable metal such as copper and nickel are attached in per seconventional manner to said film-forming lead wire 2 and saidcounterelectrode 10, respectively.

It has been discovered according to the present invention that a binaryalloy of titanium and aluminum and ternary alloy of titanium, zirconiumand aluminum can fonn a dielectric oxide film having a low leakagecurrent at high voltages when anodically oxidized.

Operable alloy compositions consist essentially of base materialselected from those defined by and included with polygonal area ABCDEFGof the diagram of FIG. 3 in accordance with the invention. Particularly,die perferable alloy compositions consist essentially of base materialselected from those defined by lines connecting points CHI of thediagram of FIG. 3, in accordance with the invention.

The alloy in the preferred composition is easily crushed into fineparticles which are useful for making a capacitor having a porouselectrode, in accordance with the invention. The atomic percentage ofthe three components of compositions ABCDEFGHI are as follows inaccordance with the invention.

TABLE 1 Ti Zr A] A.. 50 0 50 13.. so 10 70 (3-. 15 15 70 D.- 46 10 45E.. to so as F. 2o 00 20 G an 0 20 H. 2B 20 50 I....- It! 88 38 Thealloys described herein can be made in a per se conventional andsuitable manner, for example, by a vacuum melting of a mixture in agiven composition or by an arc melting of mixture of a given compositionin argon or helium atmosphere.

Said alloy can be anodically oxidized in any aqueous solution per sewell known in the art but is preferably anodically oxidized by employinga bath composition of aqueous solution containing of 0.l to l0 percentammonium phosphate in a monobasic form (Nl-LHJ'O.) or a dibasic form((Nl-LhHPO,

in accordance with the invention. Table 2 indicates the electricalproperties of anodized films on the alloys according to the inventionwhich are obtained by are melting method in an argon atmosphere.So-produced alloys in a shape of button are sliced into plates. Thealloy plates are pol'nhed electrochemically and are anodically oxidizedin a bath composition of 10 percent by weight of ammonium phosphate in adibasic form at a direct current voltage of 60 v. for 20 minutes.Capacitance and dissipation factor are measured at "1., employing acapacitance bridge. leakage current is measured by immersing theanodically oxidized alloy into an aqueous solution of 10 percent byweight of ammonium phosphate in a dibasic form after application of adirect current voltage as large as two thirds of die formation voltagefor about 2 minutes. The term leakage factor" is expressed by leakagecurrent divided by the capacitance and the measuring voltage.

TABLE 2 Composition (at. percent) Cspeei- Leakage lance DJ. actor Ti ZrAl l/cm (percent) ah-L v.)

10 45 45 0. 13 3. 5 0. 088 ll 15 70 0. i3 3. 0. 008 16 10 Q 0. i5 3. 20. 018 an 20 ll) 0. 17 0.08 0. 013 40 50 0. i7 1. b 0. 011 I) Z) 00 0.i. 2 0. 002; 25 25 50 0. i4 1. 4 011111 2) so 50 0. i4 0. 74 0. 0030 4610 46 0. 16 2. 8 0.040 60 10 40 0. 16 2. 5 0. 036 40 I) 40 (L 15 2. 0 0.040 It 33 I3 0. 10 l. 6 0. 0M3 60 3 25 0. i6 1. 1 0. 035 25 50 I 0.15 1. 3 0. 0045 16 so 25 0. 10 1. 5 0. (X168 60 30 21 0. 16 1. 3 0. 0024so 60 an 0. 15 1. 1 0. 0018 I! 0 I) 0. 16 0. 7b 0. 0083 70 0 I!) 0. i50. 83 0.01133 00 0 40 0. 15 2. 8 0. 016 I0 0 50 0. 14 1. 2 0. 043

A porom intered body of titanium-aluminum alloy ortitanium-n'rconium-alutninum alloy can be made by sintering a compactedtitanium-aluminum alloy powder or a compacted Mixtures of titaniumparticles and titanium-aluminum alloy (TiAl,) particles in compositionlisted in Table 3 are pressed at about 1850 kgJcm. into a tablet of 3mm. diameter and about 3 mm. height. The particle sizes of usedparticles are smaller than 75p. The pressed tablet is provided with afilmforming lead wire made of niobium and is heated at a temperature ofl 150 C. in a reducing pressure of 10" mm. Hg. of air for 20 minutes.The sintered body has a porosity of about 30 percent and is anodicallyoxidized in a 1 percent NHJ-TJO, aqueous solution of 25 C. at a voltageof 60 volts for 60 minutes. No pretreatment is employed.

An example of electrical properties of anodic oxide films on the poroussintered bodies mentioned above are listed in Table 3. The electricalproperties are measured in a l0 percent Nl-LHJO. aqueous solution in away similar to that described above.

The sintered body has a porosity of about percent and oxidized inelectrolytes at various temperatures and in various concentrationslisted in Table 4 at a voltage of 60 volts for 60 minutes. Nopretreatment is employed.

An example of electrical properties of anodic oxide films on the poroussintered bodies are listed in Table 4. The electrical properties aremeasured in a similar manner described above.

TABLE 4 Alloy composition Leakage (at. percent) sintering BathCapacitance iaetnr A temp. temp. Anodizing D.F. atv. Ti Zr Al C.) C.)electrolyte a! uL/cm (percent) aJaL-v.)

25 25 1, 250 96 0.1% NHiHzPOi. 4. 0b 196 19. 2 0. 0020 25 25 50 1,250 1%NiilHZPOl... 5.07 230 23.1 0.0002 215 25 50 1,250 95 1% NBA-111 0..-3.48 186 20.6 0.0016 2b 25 50 1,250 25 107 NH4H2POI 4.68 190 24.5 013330 30 40 1,150 95 0.1 NHdIzPOr 0. 14 272 15.5 0. 010 30 30 40 1,150 as1%, NHHgP04.... 7. 08 810 17.2 0.015 30 30 40 1,150 96 1 o NH4H2P04 (i.14 258 16. 5 0. (X178 34 33 33 1,160 05 0.1% NH4H2POL 3.04 157 9. 00.040 34 33 3 1,150 25 1% NH4H2PO| r 3. 92 166 11.6 0. 18 34 33 33 1,16095 1% NH4HP0| 3.60 no 10. 4 0. 085 34 33 33 1, 150 96 10% NHiHaPOr... 2.46 152 6. 0 (I. 005 34 33 38 1,160 95 10% (NHORHPOA-.. 263 155 0. 30.014

titanium-u'rconium-aiuminum alloy powder at a temperature of 800 to1300" C. in a nonoxidizing atmosphere. The powder of titanium-aluminumalloy or titanium-zirconium-aluminum alloy according to the invention isobtained by the process known in the titanium or zirconium powderpreparation.

The titanium-aluminum alloy or the titanium-zirconiumaluminum alloy isconverted into a brittle hydride compound by heating at a temperature ofabout 500 C. in an atmosphere of hydrogen. The hydride compound iscrushed into fine particles by a ball mill or another powderingapparatus. Said hydride compound is dehydrogenated by heating in anevacuated furnace tube at about 600 C.

Both titanium-aluminum binary alloys containing 50 io 75 at. percentaluminum and titanium-zirconium-aluminum ternary alloys which consistessentially of base material selectedfromtboaeddnedbylinesconnectingpointsClflofthediagram of FIG. 3, arevery brittle themselves. Therefore, the alloys in the composition rangedescribed above are easily crushed without the hydridecrush-dehydrideprocess. A porous body of titanium-aluminum alloy in the compositionaccording to th! invention can be also made by sintering a compactedmixture of titanium powder and titaniumaluminum alloy powder containing50 to 75 at. percent a1u-.

The anodized porous sintered bodies are put into containers filled withan electrolytic solution such as phosphoric acid, or ammonium phosphatesolution and sealed with a plug to prevent the electrolytic solutionfrom leaking. Construction of a wet-type electrolytic capacitor is shownin FIG. 1.

It has been discovered according to the invention that the dielectricoxide films formed on the alloy of titanium, zirconium and aluminum canform a so-called solid electrolytic capacitor superior in leakage factorwhen combined with a manganese dioxide layer integrated thereon.

Said manganese dioxide layer can be applied to said dielectric oxidefilm in a manner well-known in a prior solid electrolytic capacitorpreparation technique. For example, an alloy according to the inventionis anodically oxidized in a manner similar to that mentioned above andis immersed in a manganese nitrate solution and is heated about 250 C.to be provided with a manganese oxide layer.

Said manganese dioxide layer is coated with a carbon film layer byapplying carbon ink containing colloidal carbon. Finally a silver paintas a counter electrode is applied to said carbon film layer in aconventional method.

Table 5 shows electrical properties of solid electrolytic capacitorswhich have porous sintered anode.

minum. The capacitance and the dissipation factor are measured TABLE 3TABLE 5 (ornpositizgr c m Lam Cpnzpositior; \a pereen apac nee e a.percent age D.F. [actor Capacitance D.F. factor Ti Al Llpeilet L/cm.(percent) (pl-lflv'.) N 0. Ti Zr A1 41.) (percent) a/ i. -v.)

using a capacitance bridge. The leakage current is measured by applyinga DC voltage of H5 v across the two leads.

So-called solid electrolytic capacitors comprising a porous anode have aleakage factor of about 0.004 to 0.5 [LA./[J.F.V. at an applied voltageof "iv. in the composition range according to the present invention.

Furthermore, a leakage factor of a solid electrolytic capacitor havingan electrode made of the ternary alloy 34 at, percent titanium, 33 at.percent zirconium and 33 at. percent aluminum reaches 0.0012 MJpI-v at25 v. when the electrode formed at 75 v. is prepared by preventing fullyfrom gas element absorption during both alloy production and sinteringWhat I claim is:

l. An electrolytic capacitor comprising, in combination, a pair ofelectrodes and electrolyte means in contact with both electrodes, atleast one of said electrodes comprising an alloy consisting essentiallyof the film forming metals stated in the diagram of FIG. 3 and withinthe proportions defined by the polygonal area ABCDEFG of the diagram ofsaid figure, said one electrode having a dielectric metal oxide filmformed thereon.

2. An electrolytic capacitor according to claim I, wherein said oneelectrode consists essentially of the film forming metals defined bylines connecting points Ci" of the diagram of FIG. 3.

3. An electrolytic capacitor according to claim I, wherein saidelectrolytic means comprises manganese dioxide in contact with said pairof electrodes.

4. An electrolytic capacitor according to claim 1, wherein saidelectrolytic means comprises an aqueous solution of ammonium phosphate.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent NO.3,599,053 Dated August 0 1971 Inventor(s) Kn'rpak'i Naka+@ and Vnqh-inT'ifl:

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

Change the order of inventors as follows:

Inventors Koreaki Nakata,

Nishinomiya-shi; Yoshio Iida Osaka-fu, both of, Japan Table 1,Composition D and Table 2, Example 9,

Change 45 1O 45 to 10 45 45 Table 2, Example 1, "10 45 45" should read20 10 70 Signed and sealed this 6th day of June 1972.

(SEAL) Attest:

EDWARD M.FLETCHI1:R,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents M PO-1 0 (10-691 USCOMM-DC 60375-P69 U S, GOVEQNHQNT PRINTINGOFFICE: 1959 C355-334

2. An electrolytic capacitor according to claim 1, wherein said oneelectrode consists essentially of the film forming metals defined bylines connecting points CHI of the diagram of FIG.
 3. 3. An electrolyticcapacitor according to claim 1, wherein said electrolytic meanscomprises manganese dioxide in contact with said pair of electrodes. 4.An electrolytic capacitor according to claim 1, wherein saidelectrolytic means comprises an aqueous solution of ammonium phosphate.